Negative electrode active material for secondary battery and method for manufacturing same

ABSTRACT

Provided is an anode active material for a secondary battery and a method of fabricating the anode active material. A silicon-based active material composite according to an embodiment of the inventive concept includes silicon and silicon oxide obtained by oxidizing at least a part of the silicon, and an amount of oxygen with respect to a total weight of the silicon and the silicon oxide is restricted to 9 wt % to 20 wt %.

TECHNICAL FIELD

The inventive concept relates to a secondary battery technology, andmore particularly, to an anode active material for a secondary batteryand a method of fabricating the anode active material.

BACKGROUND ART

Secondary batteries are rechargeable and dischargeable by using anelectrode material having excellent reversibility, and lithium secondarybatteries have been commercialized representatively. Lithium secondarybatteries are expected to be provided in moveable units such as vehiclesor to be applied as medium and large sized power source used in a powerstorage of a power supply network such as a smart grid, as well as smallsized power source of small information technology (IT) appliances suchas smart phones, portable computers, and electronic paper.

When lithium metal is used as an anode material of a lithium secondarybattery, dendrites may be formed, and thereby causing shorting of thebattery or a risk of explosion. Thus, instead of using the lithiummetal, crystalline carbon such as graphite and artificial graphite orcarbon based active material such as soft carbon or hard carbon having atheoretical capacity of 372 mAh/g and capable of intercalating anddeintercalating lithium ions has been mainly used as an anode. However,as applications of secondary batteries have increased, demands forsecondary batteries having high capacity and high output have increasedmore, and accordingly, non-carbon based anode materials capable ofgenerating an alloy with lithium, for example, silicon (Si), tin (Sn),or aluminum (Al) having a capacity of 500 mAh/g or greater that mayreplace the theoretical capacity of the carbon based anode material,have drawn attention.

Among the above non-carbon based anode materials, silicon has atheoretical capacity of about 4200 mAh/g that is the largest among thosematerials, and thus, applications of silicon are considered to beimportant in view of capacity. However, since silicon expands about fourtimes greater in volume during a charging operation, an electricconnection between active materials may broke or an active material maybe isolated from a current collector due to a volume variation duringcharging and discharging processes, and an irreversible reaction such asforming of a solid electrolyte interface (SEI) may occur and lifespanmay degrade because of an erosion of the active material due to anelectrolyte. Therefore, there is a barrier in commercializing thesilicon as the anode material.

Therefore, in order to apply a silicon material, it is necessary torestrain the volume variation during the charging and discharging and toimprove an irreversible capacity of a battery. In addition, as demandsfor secondary batteries explosively increase, it is necessary to ensurea fabricating technology capable of massively producing silicon anodeactive materials.

DISCLOSURE OF THE INVENTION Technical Problem

The inventive concept provides an anode active material capable ofimproving an irreversible capacity and reducing a volume variationduring charging and discharging to have a high energy density, highcapacity, and longer lifespan, by using silicon.

The inventive concept provides a method of economically, rapidly, andmassively fabricating a silicon anode active material having the aboveadvantages.

Technical Solution

According to an aspect of the inventive concept, there is provided asilicon-based active material composite including: silicon and siliconoxide formed by oxidizing at least some of the silicon, wherein anamount of oxygen with respect to a total weight of the silicon and thesilicon oxide is restricted to 9 wt % to 20 wt %.

In one embodiment, the silicon-based active material composite mayinclude a core of the silicon, and a shell of the silicon oxide forsurrounding the core. The shell of the silicon oxide may have athickness ranging from 2 nm to 30 nm. Preferably, the shell of thesilicon oxide may have a thickness ranging from 3 nm to 15 nm. Inanother embodiment, the silicon-based active material composite mayinclude a silicon matrix and the silicon oxide dispersed in the siliconmatrix.

An average diameter of the silicon-based active material composite mayrange from 30 nm to 300 nm. Preferably, an average diameter of thesilicon-based active material composite may range from 30 nm to 200 nm.In one embodiment, a conductive layer may be formed on an outer portionof the silicon-based active material composite. The conductive layer mayinclude an amorphous carbon layer or conductive metal oxide particles.

According to another aspect of the inventive concept, there is provideda method of fabricating a silicon-based active material composite, themethod including: providing silicon particles; and oxidizing the siliconparticles to form a silicon-based active material composite includingsilicon and silicon oxide obtained by oxidizing at least a part of thesilicon, wherein an amount of oxygen with respect to a total weight ofthe silicon and the silicon oxide is restricted to 9 wt % to 20 wt %.

In one embodiment, the oxidizing of the silicon particles may beperformed by chemically oxidizing the silicon particles in a liquidsolvent containing oxygen. The liquid solvent containing oxygen mayinclude methanol, isopropyl alcohol (IPA), hydrogen peroxide (H₂O₂),water, or a mixed solvent including two or more thereof.

In another embodiment, the oxidizing of the silicon particles may beperformed by implanting oxygen ions into the silicon particles. In thiscase, the method may further include performing a thermal treatment at alow temperature of 50° C. to 200° C. for combining a silicon matrix andimplanted oxygen while excluding a possibility of thermal oxidation ofthe silicon.

Advantageous Effects

According to an aspect of the inventive concept, a composite of puresilicon, which includes silicon and silicon oxide obtained by oxidizingat least a part of the silicon and contains oxygen, an amount of whichis restricted to 9 wt % to 20 wt % with respect to a total weight of thesilicon and the silicon oxide, is manufactured in order to provide asilicon-based active material composite capable of improving lifespanand reliability while maintaining a capacity thereof at 80% or greaterwith respect to a theoretical capacity of silicon.

In addition, according to an aspect of the inventive concept, a methodof economically fabricating a silicon-based active material composite isprovided, and thereby obtaining massively the silicon-based activematerial composite having the above advantages by oxidizing siliconparticles in a liquid solvent containing oxygen or oxidizing silicon byimplanting oxygen ions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are cross-sectional views respectively showing asilicon-based active material composite according to various embodimentsof the inventive concept; and

FIG. 2 is a flowchart illustrating a method of fabricating asilicon-based active material composite according to an embodiment ofthe inventive concept.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the inventive concept will bedescribed in detail with reference to accompanying drawings.

Embodiments of the inventive concept will be described more fully withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art.

In the drawings, lengths and sizes of layers and regions may beexaggerated for convenience of description and clarity, and likereference numerals in the drawings denote like elements. As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

The terms used in the present specification are merely used to describeparticular embodiments, and are not intended to limit the inventiveconcept. An expression used in the singular encompasses the expressionof the plural, unless it has a clearly different meaning in the context.In the present specification, it is to be understood that the terms suchas “comprise” and “comprising” are intended to indicate the existence ofthe features, numbers, steps, actions, components, parts, orcombinations thereof disclosed in the specification, and are notintended to preclude the possibility that one or more other features,numbers, steps, actions, components, parts, or combinations thereof mayexist or may be added.

FIGS. 1A and 1B are cross-sectional views respectively showingsilicon-based active material composites 100A and 100B according tovarious embodiments of the inventive concept.

Referring to FIGS. 1A and 1B, the silicon-based active materialcomposites 100A and 100B each has a grain structure. The silicon-basedactive material composite 100A of FIG. 1A includes a silicon core 10 anda shell 20A surrounding the silicon core 10 and including silicon oxide.As shown in FIG. 1B, the silicon-based active material composite 100Bmay include a silicon matrix 10 and silicon oxide 20B distributed in thematrix 10.

In some embodiments, the silicon-based active material composite 100Amay further include a conductive layer 30 at the outermost portionthereof. The conductive layer 30 may further include a carbon-basedconductive layer such as graphite, soft carbon, or grapheme. Theconductive material 30 is provided for electric connection between thesilicon-based active material composites 100A contacting each other, andfor reducing an internal resistance in a current collector (not shown).

In some embodiments, the carbon-based conductive layer may be acrystalline carbon layer or at least partially amorphous carbon layer.If a carbon-based conductive layer has a high crystalline property, itmay be graphite, but in this case, a surface thereof may react with anelectrolyte. However, a low-crystalline or an amorphous carbon layer hasa chemical corrosion resistance with respect to the electrolyte,decomposition of the electrolyte is restrained during charging anddischarging, and thereby improving lifespan of an anode. Also, thecarbon-based conductive layer may have SP₂ graphite structure havingconductivity and SP₃ diamond structure having an insulating propertytogether. In order for the carbon-based conductive layer to haveconductivity, SP₂ may have a mole fraction that is greater than that ofSP_(3,) and the mole fraction may be adjusted through a thermaltreatment process.

The above carbon-based conductive layer is an example, and the inventiveconcept is not limited thereto. For example, the outermost portion ofthe silicon-based active material composite 100A may include nano-scaleparticles of conductive metal oxide such as antimony tin oxide orantimony zinc oxide, or another conductive layer such as a layer of thenano-scale particles. Although not shown in FIG. 1B, the conductivelayer 30 may be additionally provided on the silicon-based activematerial composite 100B.

The silicon-based active material composites 100A and 100B containoxygen, an amount of which is limited to a range of 9 wt % to 20 wt %with respect to a total weight of silicon forming the silicon core 10and the silicon matrix 10 and the shell 20A including silicon oxide andthe distributed silicon oxide 20B. Within the above range of the oxygenamount, the silicon-based active material composite having an initialcharging rate and a capacity maintenance characteristic, both of whichare maintained at 80% or greater, and suitable for commercialization maybe provided. The silicon core 10 and the silicon matrix 10 may includeprimary particles, but may include secondary particles obtained fromagglomeration of the primary particles. In this case, the amount ofoxygen with respect to the total weight of the silicon and silicon oxidein the silicon-based active material composite is 10 wt % to 20 wt %.

The amount of oxygen is measured in an infrared ray detection method byusing a commercialized element analyzer (ELTRA ONH-2000). In moredetail, oxygen existing in a sample is converted into carbon dioxide byusing a sample amount of 2 mg to 10 mg, calories of 8 kW, and helium(purity 99.995%) as a carrier gas, and then, a generation amount ofcarbon dioxide is measured to quantize the oxygen amount.

Table 1 below illustrates an initial efficiency and a capacitymaintenance rate of a half cell according to an amount of oxygen,wherein the half cell has an anode manufactured by using thesilicon-based active material composite according to the embodiments ofthe inventive concept. The capacity maintenance rate was measured afterperforming charging/discharging 50 times. An initial capacity thatbecomes a reference was 4200 mAh/g, that is, a theoretical capacity ofsilicon, and a power to weight ratio of the composite, which wasmeasured according to the amount of oxygen, was illustrated.

TABLE 1 Battery characteristics Initial charging/ Power to weightRetention O wt % discharging efficiency ratio @ 50 times 5 92% 2,800mAh/g 40% 7 90% 2,700 mAh/g 64% 9 90% 2,300 mAh/g 83% 10.00 89% 2,200mAh/g 84% 15.00 84% 1,900 mAh/g 97% 20.00 81% 1,700 mAh/g 98% 25.00 75%1,500 mAh/g 97% 30.00 62% 1,100 mAh/g 101% 31 50%   730 mAh/g 99% 35 53%  620 mAh/g 102%

When the amount of oxygen is less than 9 wt %, an effect of restrainingvolume expansion is insufficient, and thus, the capacitance maintenancerate of the silicon-based active material composite is reduced to 80% orless and lifespan deterioration due to the volume variation may not beaddressed. However, when the amount of oxygen exceeds 20%, although thecapacity maintenance rate is improved, the initial charging/dischargingefficiency is reduced to 80% or less and an energy density degrades.

In the silicon-based active material composite, the silicon oxide mayrestrain the irreversibility according to the charging and dischargingto improve the lifespan, by providing a tool that is capable ofabsorbing the stress caused by the volume variation of the siliconaccording to the charging and discharging. However, the silicon oxidehas a lower capacity than that of silicon, and thus, an amount ofsilicon oxide has to be limited as illustrated in Table 1 above. Thesilicon oxide applied as a substitute for addressing the high volumeexpansion rate of silicon reduces the energy density due to an excessiveoxygen amount, but in the silicon-based active material compositeaccording to the embodiment of the inventive concept, the amount ofoxygen is adjusted as described above in order to reduce theirreversibility caused by the capacity and volume variation, and therebyallowing the silicon-based active material composite to be applied asthe anode active material.

The above described capacity and the capacity maintenancecharacteristics of the silicon-based active material composite areidentified to be dependent upon sizes of the silicon-based activematerial composites 100A and 100B having the grain structures. Thesilicon-based active material composites 100A and 100B may have anaverage grain size in a range of 30 nm to 300 nm, and preferably, arange of 30 nm to 200 nm. When the average grain size is less than 30nm, a relative ratio of the conductive layer or a conductive material ofthe particle type in an active material slurry increases, and then, abattery capacity is reduced less than 80%. In addition, when the averagegrain size exceeds 300 nm, a thickness of the shell including siliconoxide increases in a case of the composite 100A of the core-shell type,and thus, the capacity is greatly reduced to 50% or less even though theirreversibility according to the volume variation is improved. It isestimated that when the thickness of the shell including silicon oxideincreases as described above, silicon oxide is more involved inoxidation and reduction of lithium even if the density of silicon oxideis appropriately controlled, and thus, it is difficult for the siliconcore to act as the active material.

In one embodiment, in the silicon-based active material composite 100Aof the core-shell structure, the shell 20A has a thickness of 2 nm to 30nm, and within the above range, the silicon-based active materialcomposite 100A has an initial capacity of 80% or greater. Preferably,the shell 20A of the silicon-based active material composite 100A of thecore-shell structure has a thickness of 3 nm to 15 nm, and within theabove range, the silicon-based active material composite 100A has aninitial capacity of 90% or greater. When the thickness of the shell 20Ais less than 2 nm, the volume expansion of the silicon core 10 may notbe prevented due to a low mechanical strength, and when the thicknessexceeds 30 nm, the shell 20A screens the silicon core 10 therein, andthereby resulting in capacity reduction.

FIG. 2 is a flowchart illustrating a method of fabricating asilicon-based active material composite according to an embodiment ofthe inventive concept.

Referring to FIG. 2, silicon particles are prepared (S10). The siliconparticle may be polysilicon or single crystalline silicon coarseparticle, and moreover, may be amorphous particle having low crystallineproperty. The coarse particles may become nano-particles through agrinding process or a polishing process, or a silicon material of alarge volume, for example, silicon load or wafer, may be electricallyexploded to prepare the silicon particles. The silicon particles arecontrolled so that a silicon-based active material composite formedthrough a process of forming silicon oxide that will be described latermay have an average grain size ranging from 30 nm to 300 nm, and morepreferably, from 30 nm to 200 nm.

The silicon particles that are miniaturized are oxidized to form asilicon-based active material composite including silicon and at least apart of silicon that is oxidized (S20). Oxidation of the siliconparticles accomplishes by the thermal oxidation. However, the abovethermal oxidation is likely to induce oxidation reaction of siliconunder a thermal equilibrium state, and thereby forming dense siliconoxide (SiO₂ that substantially satisfies stoichiometry). However, evenif such above silicon oxide that is dense and thermally oxidizedrestrains the volume variation of the silicon core and maintains thecapacity maintenance rate at 80% or greater, the silicon oxide mayscreen the silicon core therein, and thus, it is identified that thecapacity may be rapidly reduced to 60% of a theoretical capacity ofsilicon or less.

Therefore, in one embodiment, oxidation of the silicon particles may beachieved by chemically oxidizing the silicon particles within a liquidsolvent containing oxygen. The silicon-based active material compositeformed as above has an amount of oxygen restricted within a range of 9wt % to 20 wt % with respect to total weight of silicon and siliconoxide. The liquid solvent containing oxygen may be methanol, isopropylalcohol (IPA), hydrogen peroxide (H₂O₂), water, or a mixed solventincluding two or more thereof, and more preferably, water having lessenvironmental load.

Methanol is hydrocarbon having the largest amount of oxygen with respectto carbon, and restrains generation of carbon component that may occurin other hydrocarbon. Thus, methanol is advantageous for forming thesilicon-based active material composite having the silicon core and thesilicon oxide shell formed on the core. Actually, in other hydrocarbon,generation of silicon oxide on the silicon core may be interfered or anadditional thermal treatment is necessary for removing carbon in orderto form the silicon oxide, and dense SiO₂ is formed due to the thermaloxidation.

In another embodiment, the silicon-based active material composite maybe manufactured by an oxygen ion implantation process for injectingoxygen into silicon particles that are miniaturized. The siliconparticles become a silicon matrix, and ion-implanted oxygen providessilicon oxide distributed in the silicon matrix. Ion implantation energyand density in the ion implantation process are adjusted so that anamount of oxygen is limited within a range of 9 wt % to 20 wt % withrespect to total weight of silicon and silicon oxide in thesilicon-based active material composite. In order to combine the siliconmatrix with implanted oxygen while restraining densification due to thethermal oxidation of silicon, a thermal treatment may be additionallyperformed at a low temperature of 50° C. to 200° C.

In another embodiment, the silicon coarse particles undergo a grindingor a polishing process, and at the same time, the silicon particles arechemically oxidized by at least one of compressive stress and shearingstress induced from the above process to provide a silicon-based activematerial composite. If a slurry of the silicon particles is formed byusing the liquid solvent containing oxygen and the grinding andpolishing processes are performed on the slurry, the particles areminiaturized to increase a sensitivity with respect to stress, and thuschemical oxidation of the silicon particles may be induced.

Also, a process of forming a conductive layer on the silicon-basedactive material composite may be further performed (S30). A solution inwhich a conductive material is distributed together with a binder in anappropriate solvent is manufactured, and then, the silicon-based activematerial composite is dispersed in the solution to be obtained anddried, and then, the conductive layer is provided. Alternatively,polyacrylonitrile (PAN), polyethylene (PE), or a polymeric precursormaterial such as polyacrylic acid (PAA) and polyvinylpyrrolidone (PVP)is dissolved in an appropriate solvent, and after that, thesilicon-based active material composite is dispersed in the solvent toobtain intermediate particles wet by the polymeric precursor material.Then, the intermediate particles are dried and treated at a lowtemperature to obtain the conductive layer.

While the inventive concept has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the following claims.

1-16. (canceled)
 17. A secondary particle comprising agglomeratedprimary particles and a carbon-based conductive material forelectrically connecting the agglomerated primary particles, wherein anaverage diameter of the primary particles is 30 to 300 nm, wherein eachof the primary particles comprises a silicon core and a shell of siliconoxide surrounding the core, and wherein the amount of oxygen in theprimary particles with respect to the total weight of the silicon andthe silicon oxide in the primary particles is 9 to 20 wt %.
 18. Thesecondary particle of claim 17, wherein the average diameter of theprimary particles is 30 to 200 nm.
 19. The secondary particle of claim17, wherein the shell of silicon oxide has a thickness of 2 to 30 nm.20. The secondary particle of claim 17, wherein the shell of siliconoxide has a thickness of 3 to 15 nm.
 21. The secondary particle of claim17, wherein the amount of oxygen in the primary particles with respectto the total weight of the silicon and the silicon oxide in the primaryparticles is 10 to 20 wt %.
 22. The secondary particle of claim 17,wherein the silicon core has a low crystallinity.
 23. The secondaryparticle of claim 17, wherein the silicon oxide shell is formed on thesilicon core of each of the primary particles by chemically oxidizingsilicon particles by grinding a slurry of the silicon particles in aliquid solvent comprising oxygen.
 24. The secondary particle of claim17, wherein the carbon-based conductive material is formed by dispersingthe primary particles in a solvent comprising a polymeric precursor ofthe carbon-based conductive material to obtain intermediate particleswet by the polymeric precursor, drying the intermediate particles, andheat treating to obtain the carbon-based conductive material.
 25. Thesecondary particle of claim 17, wherein the carbon-based conductivematerial is formed of crystalline carbon, soft carbon, graphite,graphene, or amorphous carbon.
 26. The secondary particle of claim 17,wherein the carbon-based conductive material has a greater mole fractionof sp² carbon than sp³ carbon.
 27. The secondary particle of claim 17,wherein the carbon-based conductive material is a layer at the outermostportion of the primary particles.
 28. The secondary particle of claim27, wherein the carbon-based conductive material is formed ofcrystalline carbon, soft carbon, graphite, graphene, or amorphouscarbon.
 29. The secondary particle of claim 28, wherein the carbon-basedconductive material has a greater mole fraction of sp² carbon than sp³carbon.